Characterization of serotonin in protein and membrane mimetic environments: a spectroscopic study

As a first step toward using the photophysical properties of serotonin to probe its interactions with biological target sites, we have examined its interactions with human serum albumin (HSA), chosen as a surrogate for the actual receptor proteins in physiological systems, and with sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/heptane/water reverse micelles, chosen as a biomembrane mimetic environment for the transmembrane portion of the receptor protein. Although the emission maximum of serotonin is relatively insensitive to the polarity of the local environment, which is attributed to lack of solvent dipolar reorientation of the 5-hydroxyindole chromophore, the fluorescence anisotropy (r) served as a useful and sensitive parameter from which the binding constants (K) and Gibbs energy changes (deltaG) were estimated for serotonin-HSA and serotonin-AOT reverse micellar interactions. Fluorescence-decay studies of serotonin show double-exponential kinetics in homogeneous aqueous solvent due to the structural heterogeneity arising from different rotamers of serotonin. In contrast, upon binding to HSA, a single-exponential fluorescence-decay profile was observed indicating the occurrence of a single structural species of serotonin in the protein environment. Furthermore, far-UV-circular-dichroism (CD) spectroscopic data indicate that the secondary structural features of HSA remain essentially intact after binding to serotonin. This preliminary research can be expected to open the door to extensive future studies on interactions of serotonin with relevant target proteins and associated cell membranes involved in its diverse physiological functions.